The interweb is almost awash with horror stories of how engines leak after using internal cleaning solutions - I'm surprised you haven't noticed!
If you've got a long time sitting engine I reckon the best thing to do is to drain the gunky muck you've got. If you are brave enough remove the sump and give it a good scrape - clean with brake cleaner / other external engine cleaners (Gunk seems popular in the UK and is available from easy to get to places like Halfords). Make sure the sump is clean and dry - refit.
Fill the engine with the correct grade engine oil but buy some cheap junk (also available from aforementioned place) and use this for a few days (don't leave it in for a full service interval!). Then drain (now on a hot engine) and put in some decent stuff.
(Seeing as how cheap filters are I'd replace the filter each time)
This is the same approach I have for gearboxes and differentials. Gravity has a really good effect on long sitting machines - heavy muck does in the end settle at the lower points. Removing this muck from the system before spinning everything back up again is advantageous.